An embodiment of the present invention is a method and apparatus for the acquisition of an image by nuclear magnetic resonance (NMR). The acquired image is that of a part of an object, such as body, subjected to an intense magnetic field known as an orienting field during a nuclear magnetic resonance procedure. This type of procedure is meeting with growing success in the field of medicine where the images produced serve as a diagnostic aid, especially in cardiac diagnostics.
However, the application of the disclosed method and apparatus is not restricted to this field. The method and apparatus can also be implemented, for example, in the field of physical measurements where spectrometers are used or for inanimate and animate objects.
An embodiment of the invention enables the differentiation, in the heart which is an organ in constant movement, of signs of infarction or, in the context of heart transplants, of signs of rejection, especially well before such rejection become pathological.
In nuclear magnetic resonance imaging, to obtain an image of a slice of an object to be examined, the object in question, and especially the part in which the slice is located, is subjected to a constant, intense and uniform magnetic field B0. Under the effect of this field B0, the magnetic moments of the particles of the body are oriented within a few instants (within a few seconds), in the direction of the magnetic field: hence the name “orienting field” given to this field. If the magnetic moments of the particles of the object are then excited with an RF magnetic excitation oscillating at an appropriate frequency, this causes the orientation of the excited magnetic moments to flip. At the end of the excitation, the magnetic moments tend to get realigned with the orienting field in a natural precession motion known as a free precession motion. During this precession motion, the particles radiate and an electromagnetic de-excitation energy that can be measured. The frequency of the de-excitation signal, also known as NMR, is characteristic of the excited particles (in medicine, this is the hydrogen atom contained in the molecules of water or fat or other compounds spread throughout the human body) and of the strength of the orienting field. The characteristics of the object can be deduced from the processing of the measured signal.
The processing of the measured signal to extract an image is complicated by the fact that all the particles of the object, throughout the excited region, re-emit a de-excitation signal at the end of the excitation. It is therefore important to distinguish the contributions, in the total NMR signal, of all the elementary regions (known as voxels) of the excited volume to reconstruct their distribution, and ultimately to prepare the image. This discrimination is possible only by carrying out a series of excitation-measurement sequences during each of which an encoding is done, differently from one excitation or one sequence to another, of the NMR signals to be measured. If the encoding applied is known, then there are known ways of using pure imaging techniques, especially of the 2DFT type, to rebuild the image.
The measurement of the NMR signal relates to the amplitude of this signal. Given a demodulation frequency around which the NMR signal is demodulated, all that can be hoped for as a result of the measurement is a measurement of the density, in the structures being examined, of the specific particles (e.g., hydrogen) for which only one of the resonance frequencies is then studied. Broadly speaking, at the end of a given duration of the excitation, the higher this density, the stronger is the NMR signal.
The relaxation, or damping, of the NMR signal is data used to obtain information on the object being studied. This damping is a complex damping: it represents an interaction known as a spin-lattice interaction of the particles (e.g., the proton of the hydrogen atom) excited with the surrounding matter and an interaction known as the spin-spin interaction between the protons.
In a known modeling of the physical phenomena that come into play, it has been determined that the spin-lattice relaxation time, also called the time T1, corresponds to the time constant of an exponential growing or growth (a reorientation) of the component, aligned with the orienting field (or longitudinal with it), of the total magnetic moment at the concerned place in the object. The spin-spin relaxation time, called T2, also corresponds to a time constant, in this case the time corresponding to an exponential decay of the transversal component (orthogonal to the longitudinal component) of these magnetic moments.
In one example that shall be described further below in the context of the description of an embodiment of the invention, reference shall be made to a time T1 of about 750 ms and especially a time T2 of about 30 ms to 100 ms: the concerned regions of the object will then be chiefly those of the heart. The distinction between the regions of the object subjected to lesions and the healthy regions is revealed by T2 images.
It is possible, during a series of procedures of different types, to make one relaxation phenomenon appear by preference over the other. It is then said that T1 or T2 images have been made as the case may be. The essential parameter of NMR experiments that is generally brought into play is then the repetition time TR that punctuates the periodicity of the excitation pulses in the sequences of the series of imaging sequences implemented.
To make a T2, image, it is desirable await a total growth of the magnetization (of its longitudinal component): between each sequence, it is desirable to wait for a duration of about three to four times the duration of T1. At the end of this duration, leaving aside the concentration of the particles (which is overlooked), it can be said that the first NMR signal measured is dependent only on the relaxation time T2. It is only if the repetition rate is far too rapid that the influence of the decay in T2 of the NMR gets eliminated before the size of the differentiation of the growth in T1. One of the difficulties of T2 images appears immediately at this stage of the explanation: it is that they are long. In practice, they are about three or four times longer than the T1 images. For example, for a T2 image of the heart, a patient has to be undergo the experiment in a motionless state (holding his breath in particular) for successive periods of time which could add up to a total of about 16 minutes. Although certain patients are fairly cooperative, the NMR procedure that is harmless as it happens, then becomes very painful, especially as patients are generally feeble persons.
A prior art method of overcoming the drawbacks mentioned proposes a fast T2 image which, in one example, may last up to three or four minutes while avoiding the disturbing effects of the contrast in T1 in this image. The goal to be attained is that the acquisition period should not be too lengthy, both for the patient and for the cost effectiveness of the apparatus. This duration in any case is far too lengthy for cardiac examination.
Fast image acquisition methods known as methods of the fast spin echo or FSE type have been proposed. In practice, we can distinguish sequences known as pure FSE sequences with, typically, 16 echoes and 16 acquisitions repeated every four seconds. This leads to 64 seconds of acquisition for one slice. Other methods, known as single shot FSE (SSFSE) type methods are described here below. In these SSFSE methods, the sequences used comprise an excitation of the magnetic moments of the protons, known as flip. This excitation gives rise to a major flip in magnetization, typically equal to 90°, followed by a large number of spin echo excitation pulses (at 180°), known as refocusing pulses, very close to one another and typically separated from one another by a period of about a few milliseconds. To simplify the explanation, one example will take a duration of 5 ms between each echo and the next. But naturally the embodiment of the invention can be applied to cases where the echo time is smaller or greater.
Furthermore, between each of these refocusing pulses and the next, and before the read operation, phase-encoding gradients are applied. These gradients vary in successive steps from one echo to another. The gradient pulses thus applied are furthermore compensated for in a following gradient pulse, before the next refocusing pulse. As regards the slice selection gradient pulses, the fact of centering them in time on the central date of the refocusing pulse causes them to be automatically compensated for. It can be shown that this is also the case with read gradient pulses.
Typically, it is possible in this way, in a single sequence of decay of the T2 signal (for example in a period of about 400 ms), to acquire 128 echoes. By thus applying read gradients during the reading, it is possible, at the end of a single sequence, to acquire an entire image in a very short period of time, in the range of T2 or a multiple of some T2 periods. Typically, each refocusing pulse may last 5 ms (to be very selective), or 2.4 ms in being less selective. In an embodiment of the invention, it is rather the latter duration that will be chosen, even though, for the useful 90° excitation, a lengthier duration could be envisaged. The measurement of the NMR signal at the time of each echo, in taking 256 samples every 8 microseconds, last about 2 ms.
The phase encoding sequences comprises encoding pulses whose value evolves in steps, from a negative value to a positive value. The negative or positive character is only a matter of convention. It has no meaning except to the extent that it enables the distinguishing of field gradients oriented along an axis, in one sense or another on the axis.
As is known, the effective echo time is the time between the application of the first excitation pulse, in practice the 90° pulse that creates the measurable SSFSE signal, and the time at which the phase-encoding gradient undergoes a reversal of polarity.
With the above prior art solution, we would be led to making images whose resolution would depend on the effective echo time. In the prior art, for an effective echo time of 100 ms, with an echo time between spin echo pulses equal to 5 ms, 20 encodings could be made before reversal, and 20 after, giving an image on 40 lines. This is acceptable. However, for an effective echo time of 30 ms, under the same conditions, there would be no more than 12 image lines, and this is insufficient.
Furthermore, firstly the programming of such an experiment would be fairly complex owing to the fact that, from one sequence to another, many parameters would be changed. Secondly, the growth of the magnetization, at the end of the time T1 is not fully controlled since, from one sequence to another, the duration of the sequences would be different.
It will also be noted that the duration of the sequences is limited by the absorption of energy in the patient. It must be considered in the case of multi-slice image acquisition (for which the duration of acquisition is proportional to the number of slices). The number of acquisitions thus soon becomes a problem if each acquisition lasts too long.